Examples of conventions mechanical--hydrostatic transmissions are disclosed for example in Tokai Hei. 1-250661 and Tokkai Sho 6-153055 published by the Japanese Patent Office. These transmissions however do not comprise a rotary distributor for receiving fluid pressure supplied from an external source.
In Tokkai Sho 52-5034, a mechanism is disclosed which is similar to a differential hydraulic motor. The rotation sped nY of the output shaft of this differential hydraulic motor is given by nY=nX+nH where the rotation speed of the input shaft is nX, and nH=(Q/U), where the effective capacity=U and the fluid supply flowrate from a external pump=Q. The rotation speed of a differential hydraulic motor is therefore higher than that of an ordinary hydraulic motor which rotates only due to fluid pressure.
However as this differential hydraulic motor is of such a construction that it rotates together with the cylinder block, a rotary distributor is required for supplying fluid from an external hydraulic pump, and when it is rotating at high speed, problems arise due to frictional losses in the distributor and durability. Further, as the positions in an axial direction of a plurality of pistons disposed inside cylinder block are different, a moment is generated which tilts the cylinder block due to its rotation and the sealtightness of valve plates is easily impaired.
Also, the differential hydraulic motor comprises a spool-shaped change-over valve or a cylinder type change-over valve as described in the aforesaid publications. These change-over valves operate according to the relative rotation of the input shaft and the output shaft. The pistons disposed in the cylinder block extend and contract as they slide on a swash plate. Due to these change-over valves, high pressure is supplied to the pistons as they extend and low pressure when they contract, and the cylinder block therefore rotates continuously.
However, the construction of the fluid flowpath in the spool-shaped change-over valve mentioned in the above publication is complex, and its machinability is not at all satisfactory. In the case of the cylinder type change-over valve on the other hand, sealtightness is easily impaired due to fluid pressure.